Suspect pump?

[gas4you]

You could try removing the pump to check for sludge build up in it. Unfortunately the 15/60 pump is rather expensive if you get a Grundfoss replacement, so it could be money wasted.

Could also just remove pump head to check it.

 

[Agile]

Its all very well saying that you put "descaler" in the system but unless you tell me exactly what you used and the dosage then I am unable to form a view as to the most likely cause.

The mere fact that water runs out clear means nothing other than there is no suspended dirt. There may be dirt lining the inside of the tube or agregations of bits somewhere. That often happens after chemicals are put into the system.

It may well be dirt in the pump impeller but the non invasive temperature measurements will give an indication of that ( or not ).

Tony